Igor Girkin, who is also known by his pseudonym Igor Strelkov, has been called "one of the most powerful separatist figures in eastern Ukraine." He's a Russian citizen from Moscow and has declared himself the Minister of Defense of the self-proclaimed Donetsk People’s Republic (DPR), according to Radio Free Europe.

The Ukrainian government says Strelkov is a covert agent of Russia's GRU military intelligence. In documents posted on separatist websites he has asked Russia to provide military assistance to the DPR.

Strelkov is a veteran of both the Soviet and Russian armies, according to Reuters.

He moves through the streets in a black Mercedes, his face with pencil moustache hidden behind tinted windows, and his aim is to "destroy" Ukrainian forces that venture onto his territory.

In a leaflet distributed in the rebel Donetsk region, "Colonel Igor Strelkov" assumed command of all rebel forces there and called for Russian army help to ward off what he calls the threat from the Kiev "junta" and from NATO.

Strelkov's history and his powerful position within the separatist movement in the east could be taken as proof that Russia is assisting or even coordinating the separatist uprising.

Russia denies this, but in a news conference on Friday, President Barack Obama said that the separatists "have received a steady flow of support from Russia," and Ukraine's government claims that Strelkov has been taking orders from Moscow.

Strelkov was reportedly present during the Russian annexation of Crimea, the Black Sea peninsula that Russia seized from Ukraine in March. He then moved to eastern Ukraine as the situation in that part of the country escalated.

Before he came to Ukraine, Strelkov was involved in conflicts in Yugoslavia and Chechnya, according to the BBC.

Russian media claims that he used to work for the Russian Federal Security Service within the Directorate for Combating International Terrorism.

But in April, the European Union put Strelkov on its sanctions list and described him as working for the GRU, according to Reuters.

Around the time the Malaysia Airlines plane went down, Strelkov posted a statement on Russia's largest social network that seemed to take responsibility for the attack. The post was later deleted, and it's now widely thought that the separatists mistook the Malaysia plane for a Ukrainian aircraft.

U.S. officials have said that pro-Russian separatists fired the fatal missile, but no group has claimed responsibility so far. Nearly 300 people died in the attack.